Re: [PATCH v6 1/9] x86/fred, KVM: VMX: Pass event data to the FRED entry point from KVM

[Sohil Mehta <sohil.mehta@intel.com>]

On 5/14/2025 7:15 AM, Sean Christopherson wrote:
> On Tue, May 13, 2025, Sohil Mehta wrote:
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 5c5766467a61..1d43d4a2f6b6 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7079,7 +7079,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu,
>>  
>>  	kvm_before_interrupt(vcpu, KVM_HANDLING_IRQ);
>>  	if (cpu_feature_enabled(X86_FEATURE_FRED))
>> -		fred_entry_from_kvm(EVENT_TYPE_EXTINT, vector);
>> +		fred_entry_from_kvm(EVENT_TYPE_EXTINT, vector, 0);
>>  	else
>>  		vmx_do_interrupt_irqoff(gate_offset((gate_desc *)host_idt_base + vector));
>>  	kvm_after_interrupt(vcpu);
>> @@ -7393,7 +7393,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>>  	    is_nmi(vmx_get_intr_info(vcpu))) {
>>  		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
>>  		if (cpu_feature_enabled(X86_FEATURE_FRED))
>> -			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR);
>> +			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR,
>> +					    vmx_get_exit_qual(vcpu));
>>  		else
>>  			vmx_do_nmi_irqoff();
>>  		kvm_after_interrupt(vcpu);
> 
> As a prep patch, what if we provide separate wrappers for IRQ vs. NMI?  That way
> KVM doesn't need to shove a '0' literal for the IRQ case.  There isn't that much
> code that's actually shared between the two, once you account for KVM having to
> hardcode the NMI information.
> 

Seems reasonable to me.

I don't see the IRQ side using the Event data anytime soon (or ever). We
still need to pass 0 to asm_fred_entry_from_kvm() for the IRQ case but
that would be contained in asm/fred.h and would not affect KVM.

> Compile tested only...
> 

No worries. I'll test it out. I am assuming you want this patch to go as
part of this series.

> --
> From: Sean Christopherson <seanjc@google.com>
> Date: Wed, 14 May 2025 07:07:55 -0700
> Subject: [PATCH] x86/fred: Provide separate IRQ vs. NMI wrappers for "entry"
>  from KVM
> 
> Provide separate wrappers for forwarding IRQs vs NMIs from KVM in
> anticipation of adding support for NMI source reporting, which will add
> an NMI-only parameter, i.e. will further pollute the current API with a
> param that is a hardcoded for one of the two call sites.
> 
> Opportunistically tag the non-FRED NMI wrapper __always_inline, as the
> compiler could theoretically generate a function call and trigger and a
> (completely benign) "leaving noinstr" warning.
> 

If this is really a concern, wouldn't there be similar semantics in
other places as well?

> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/include/asm/fred.h | 24 +++++++++++++++++++-----
>  arch/x86/kvm/vmx/vmx.c      |  4 ++--
>  2 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
> index 2a29e5216881..dfb4f5e6a37a 100644
> --- a/arch/x86/include/asm/fred.h
> +++ b/arch/x86/include/asm/fred.h
> @@ -10,6 +10,7 @@
>  
>  #include <asm/asm.h>
>  #include <asm/trapnr.h>
> +#include <asm/irq_vectors.h>
>  

I'll move the header insertion to keep it alphabetically ordered.

>  /*
>   * FRED event return instruction opcodes for ERET{S,U}; supported in
> @@ -70,14 +71,26 @@ __visible void fred_entry_from_user(struct pt_regs *regs);
>  __visible void fred_entry_from_kernel(struct pt_regs *regs);
>  __visible void __fred_entry_from_kvm(struct pt_regs *regs);
>  
> -/* Can be called from noinstr code, thus __always_inline */
> -static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int vector)
> +/* Must be called from noinstr code, thus __always_inline */
> +static __always_inline void fred_nmi_from_kvm(void)
>  {
>  	struct fred_ss ss = {
>  		.ss     =__KERNEL_DS,
> -		.type   = type,
> +		.type   = EVENT_TYPE_NMI,
> +		.vector = NMI_VECTOR,
> +		.nmi    = true,
> +		.lm     = 1,
> +	};
> +
> +	asm_fred_entry_from_kvm(ss);
> +}
> +

The original code uses spaces for alignment. Since we are modifying it,
I am thinking of changing it to tabs.

> +static inline void fred_irq_from_kvm(unsigned int vector)
> +{
> +	struct fred_ss ss = {
> +		.ss     =__KERNEL_DS,
> +		.type   = EVENT_TYPE_EXTINT,
>  		.vector = vector,
> -		.nmi    = type == EVENT_TYPE_NMI,
>  		.lm     = 1,
>  	};
>  
Thanks,
Sohil